The present invention relates to riser apparatus used in offshore drilling applications and more particularly to riser apparatus that is specially adapted for use in deep water.
A continuing search for new sources of fossil fuels has expanded outwardly from continental land masses and their bounding shallow-water shelves out to the open sea. Understandably, a plethora of problems arise to thwart and further complicate the best laid plans that high technology can develop in the quest for deep sea oil reserves.
Not the least of these problems is due to the vagaries of nature as they relate to climatic conditions. Often there is an open period suitable for drilling followed by a period of bad weather conditions during which well drilling operations must be suspended. Depending upon the severity and duration of the weather, a suspended well may be left unattended for the duration of the season. Not only does this result in a substantial loss of revenue but, in addition, additional finances must be provided to cover the extra costs involved in suspending and resuming well drilling.
At current rates, the cost of maintaining a drillship on site runs in the order of $300,000 per day. Taking into account travel time as well as the time necessary for preparing and abandoning a drill site, the cost of an untoward delay of three days would involve a sum in excess of $1,000,000.
Another problem that is particularly troublesome in deep sea operations is the difficulty of keeping the well overbalanced as the water depth increases. For example, assuming that a well is in a comfortable position of 100 psi overbalance, should it become necessary to move off location a reduction in hydrostatic head will occur when the riser is disconnected from the well. The magnitude of the reduction will depend on the mud weight and the water depth, and will amount to the difference between the density of the mud in use and the density of sea water, multiplied by the length of riser in use. In the case of a 3000 ft. riser using 12 lbs./gal. mud, the reduction in hydrostatic head would be more than 100 psi, taking the well from a condition of 100 psi overbalance to a condition of at least 400 psi underbalance. Precautions can of course be followed to avoid losing control of the well, as by controlling the rate of penetration, accurately controlling mud weight, circulating and conditioning the mud, to name but a few. However, these precautions are observed mainly during such times when there is an anticipation of pressure zones and/or during times of bad weather which may require a well disconnect to ensure the safety of personnel.
Numerous other problems occur, all of which are depth related which adversely affect personnel safety and extend drillship operating times. For instance, the difficulties of re-entry are directly proportional to the depth of the re-entry point. The advantages of a re-entry operation in shallow water offering diver access are thus readily apparent. Furthermore, equipment simply cannot be maintained by divers at depth and serious malfunctions can lead to pulling the riser or even abandoning the well in extreme cases.
One answer to the problem of equipment maintenance is to substitute sophisticated remote controls. This, however, in an expensive alternative and is frequently inadequate to deal with the myriad of problems that may occur on site that only the human intellect and manual dexterity may solve.
The operational zones below the surface of the sea may be categorized by depth. Thus, the top 50 meters of the sea can be considered as the weather zone which can be subdivided into a splash zone (above) and a wave zone which includes the splash zone.
The top 100 meters is readily accessible to divers although diving operations are limited in the zone between 100 and 500 meters. Operations beyond 500 meters are infrequent and, for most practical purposes, not feasible.
Beyond the 500 meter depth, it is no longer feasible to use conventional hydraulic lines for actuating blow-out preventer (BOP) controls, and, as a result, resort must be made to electro-hydraulic relaying.
The problem of significant loss of hydraulic head of a riser disconnected in deep water has been noted. Assuming that a 12 lb. mud is maintaining a 100 psi overbalance, this overbalance can be lost if the water depth is greater than 170 meters. Improved well safety by keeping the BOP within 200 meters of the surface will ensure only moderate mud head loss and permits maintenance by divers if needed. This however is merely a re-statement that it is preferable to conduct drilling operations in shallow water since heretofore it was considered incongruous to associate an elevated BOP with a deep sea drill site at which the conventional position of the BOP is on the sea bed.